This invention relates generally to apparatus and methods for measuring flow rates of fluids. In particular, the invention relates to an improved vortex flowmeter. Vortex flowmeters measure the rate of flow of a fluid, termed a process fluid, by measuring the frequency of artificially induced vortices in the fluid. Such flowmeters are known in the art, and include those marketed by The Foxboro Company, Foxboro, Mass., U.S.A. (xe2x80x9cFoxboroxe2x80x9d) under the trade designation E83. Vortex flowmeters are popular because of their relatively high accuracy and wide dynamic range. In addition many vortex flow meters can operate in extreme temperatures, for example, up to 800xc2x0 F.
Vortex flowmeters typically have a tubular passage, such as a pipe, for guiding the process fluid therethrough and have an obstruction element, also termed a vortex shedder, interposed in the path of fluid flow. The obstruction element includes a bluff surface facing the fluid flow for creating a series of spaced vortices downstream in the flowing fluid. Under certain conditions, the vortex shedder creates two nearly-parallel rows of vortices on opposite sides of the shedder. These vortices are known in the art as a Von Kalman vortex street. The vortices in one row are staggered with respect to the vortices in the other row. It is understood that the frequency of these generated vortices is typically linearly proportional to the average flow velocity of the fluid. Thus, a measurement of the frequency of the vortices provides a measure of the average flow velocity. A vortex-responsive sensor detects the pressure fluctuations associated with the passage of the vortices and drives an electronic unit that determines the frequency of the vortices, to determine the flow velocity of the fluid.
In many conventional vortex flowmeters, the obstruction element spans the entire diameter of the pipe that guides the flowing fluid, and it typically forms an integral structure with the pipe. Such an integral structure does not allow easy access to the obstruction element, thereby rendering inspection of the obstruction element difficult. In addition, such a structure does not allow easy replacement of the obstruction element when such a replacement becomes desirable.
Another disadvantage of conventional vortex flowmeters having obstruction elements that span the entire diameter of the fluid-guiding pipe is that the vortex-shedding frequency for pipes having large diameters is low. For example, the vortex-shedding frequency in a vortex-flowmeter having a pipe with a diameter of approximately 12 inches (30 cm), is typically as low as 1 Hz for a flow rate of approximately 1 ft/sec, and is less than 1 Hz for larger pipes. Various sources of noise in a vortex flowmeter contribute to the noise level of a vortex-induced signal. To reduce the noise level of the signal, a vortex flowmeter typically obtains an average signal by integrating the vortex-induced signals over a number of vortices. A vortex flow meter having a low vortex frequency, however, would require a long averaging time for an effective reduction in the noise. Thus, the response time of such a flowmeter is typically slow.
Flowmeters that have flow measuring elements that are inserted into the path of a flowing fluid without spanning the entire diameter of the pipe are also known in the art. Many of such flowmeters do not rely on generating vortices for measuring the flow velocity of the fluid. For example, paddle wheel and annubar meters, and pitot tubes are known in the art. Such flowmeters have either small pressure ports that are susceptible to clogging or moving parts that tend to wear out, and thus require periodic inspection and replacement.
U.S. Pat. No. 4,562,745 of Parra teaches a vortex-type flow meter for insertion into a pipe carrying a fluid. The flowmeter of the ""745 patent includes a tubular bluff body that includes an opening bridged by a separate bluff body. Both bluff bodies produce vortices, but those produced by the tubular body cause an unwanted interference with the vortices produced by the other bluff body, thereby causing measurement errors. Fins are added to suppress the unwanted vortices. The design is cumbersome inasmuch as it employs two, rather than one, bluff bodies, and ancillary structures for minimizing interference between the vortices induced by the two bluff bodies. In addition, the tubular bluff body of the ""745 patent introduces added flow obstruction, thereby increasing pressure losses across the flow meter which in turn increases costs associated with pumping the fluid through the meter.
It is thus an object of the invention to provide a vortex flowmeter having a removable and replaceable obstruction element.
It is another object of the invention to provide a vortex flowmeter having an obstruction element that partially spans the inner diameter of a pipe for guiding the flow of a process fluid.
The invention attains the foregoing and other objects by providing an insertion-type vortex flow meter that includes a pipe having a wall that forms a conduit for guiding a flowing fluid, herein referred to as process fluid, therethrough. The pipe of the flow meter includes an opening therein that allows an obstruction element to be removably and replaceably disposed within the conduit. The obstruction element is configured to span the inner diameter of the pipe partially, and is suspended rigidly within the conduit. The flow of the fluid past the obstruction element produces two streams of vortices, with the vortices in one stream staggered or spatially offset with respect to those in the other stream. A sensor element, disposed within the obstruction element, detects the induced vortices, thereby measuring the flow velocity of the fluid.
According to one aspect of the invention, the obstruction element is connected to a support element, such as a tube, that allows easy insertion of the obstruction element into the conduit, and further allows adjustable positioning of the obstruction element at a desired distance from the wall of the pipe.
Another aspect of the invention relates to providing the obstruction element with a flow conditioning clement, such as a tubular section or end plates attached to the shedder or blunt surface, to define the boundaries of the induced vortices, which in turn helps stabilize the vortex shedding over a wide frequency range. Such a stabilization of the vortex shedding ensures production of a strong stable signal, and improves the linearity of the shedding frequency as a function of the flow velocity. Further, such flow conditioning elements advantageously ensure that the frequency of the induced vortices is substantially independent of the size and/or the shape of the conduit for guiding the fluid.
According to another aspect, the flow conditioning element has a major axis that extends axially along or within the pipe, and is coupled or attached to an obstruction element. The axially extending flow conditioning element preferably has a the pipe. This positioning and arrangement of flowmeter components eliminates the need for forming additional unwanted structure, such as stabilizing or vortex reducing/ canceling fins, on the obstruction element.
According to another practice, the flowmeter of the invention includes a first flange sub-assembly having a first flange for disposing a flow-obstruction element within an opening in the pipe. The obstruction element has a sensor element for detecting a stream of vortices induced by the flow obstruction element for thereby measuring the flow velocity of the fluid. The flowmeter also includes a second flange sub-assembly having a second flange for coupling to the first flange for mounting the first flange sub-assembly to the pipe. According to one practice, a first flange is couple to the outside surface of the pipe wall as an anchor for suspending the obstruction element rigidly within the conduit. In particular, the obstruction element is attached to a second flange that is mounted on the first flange, thereby holding the obstruction element in place within the conduit.
The invention in one aspect provides convenient removal and replacement of the obstruction element. In particular, some embodiments of the invention include a hot tap for replacement of the obstruction element without stopping the flow of the process fluid. Further, the obstruction element of the present invention can be installed in pipes having a variety of different inner diameters without machining the obstruction element to size it for a particular pipe size.
These and other features of the invention are more fully set forth below with reference to the detailed description of illustrated embodiments, and the accompanying figures in which like numerals refer to like elements.